A clause in Dalot's contract will allow the Portugal Under-21 international to leave for €20 million from June 1.

But with a host of Europe's top clubs chasing the teenager, who has only made eight senior appearances, sources have told ESPN FC that United are prepared to offer a higher fee to convince Porto to sell before the clause is activated.

Sources have told ESPN FC that Bayern Munich and Barcelona are among the clubs interested.

Jose Mourinho has targeted Dalot, who can play at right-back and left-back, as he looks to bolster his defensive options ahead of the new season.

Regular full-backs Antonio Valencia and Ashley Young are both approaching their 33rd birthdays while Matteo Darmian and Daley Blind could leave Old Trafford this summer.

Sources have told ESPN FC that, as well as Dalot, United are also keen to sign Shakhtar Donetsk midfielder Fred and Tottenham defender Toby Alderweireld before the World Cup kicks off next month while there remains interest in Chelsea winger Willian.

Mourinho has also been linked with a move for West Ham striker Marko Arnautovic and the United manager was in the stands to watch the 29-year-old -- who played under Mourinho at Inter Milan -- feature for Austria against World Cup hosts Russia in Innsbruck on Wednesday night.

Rob is ESPN FC's Manchester United correspondent. Follow him on Twitter @RobDawsonESPN.